This invention relates to a liquid crystal optical switching element, and in particular to a liquid crystal optical switching element which is suited for use for a color shutter and a color image display device.
Since a liquid crystal optical switching element is advantageous in terms of light weight and low power consumption, it has been developed and put into practical use as a display mainly for a notebook type personal computer or for a portable information processor. In view of advancements in transmitting information through multimedia in recent years, there is an increasing demand to make the most of optical switching element for an animation or for a large picture plane. In order to meet these demands for an animation or for a large picture plane, it is imperative to realize a liquid crystal optical switching element of high speed and wide viewing angle. Under the circumstances, there have been intensively studied various kinds of systems for realizing these demands, and some of which have been put into practice. Examples of systems that have been proposed up to date include a ferroelectric liquid crystal (FLC) element, an antiferroelectric liquid crystal (AFLC) element, an in-plane switching (IPS) element, a .pi. cell element, a vertically aligned (VA) element, and a bistable twisted nematic (BTN) liquid crystal element. Among them, since the ferroelectric liquid crystal (FLC) element and antiferroelectric liquid crystal (AFLC) element exhibit a spontaneous polarization, it is possible according to these elements to obtain a high response speed ranging from several microseconds to several tens microseconds. In view of the fact that the response speed according to systems other than those of FLC element and AFLC element is at most 1 ms, these FLC and AFLC elements are more advantageous in terms of response speed. The systems employing these FLC and AFLC elements however are accompanied with a serious problem that they are subject to an irreversible breakdown of alignment due to an external stress.
On the other hand, there is known, as a high speed optical shutter that has been put into practical use, a device utilizing Kerr effect. The Kerr effect is a phenomenon where an optical anisotropy is exhibited as a voltage is applied on a transparent isotropic medium, the intensity of the optical anisotropy being proportional to a square of electric field E. This Kerr effect was found out by J. Kerr in 1875. Thus, when a birefringence to be induced by an electric field is represented by .DELTA.n and a wavelength of light in vacuum is denoted by .lambda., there is a relationship between them which is represented by a formula; .DELTA.n=K.lambda.E.sup.2 wherein K is called Kerr constant. As for a material exhibiting a large Kerr constant, examples of which include a liquid such as carbon disulfide or nitrobenzene, and a solid such as PLZT (a metal oxide comprising a solid solution consisting of lead zirconate and lead titanate, into which lanthanum is further added). These materials can be utilized for an optical shutter in combination with a polarizer. For example, in the case of an optical shutter making use of an electric field of pulsed laser beam, it is possible to realize a response speed of 2 ps when carbon disulfide is employed (Appl. Phys. Lett. 26,92 (1975)), and a response speed of 32 ps when nitrobenzene is employed (Appl. Phys. Lett. 15,192 (1969)). However, since these materials are poisonous or explosive, it would be difficult to put them into practical use. On the other hand, an optical shutter employing the PLZT is now put into practical use, the response speed thereof being in the range of from 0.1 .mu.s to 10 .mu.s. The manufacture of a display element using the PLZT has also been tried as set forth in Ferroelectrics 50, 63 (1983); or SID 84 Digest, 137 (1984). However, the PLZT is inherently accompanied with a problem that the mechanical strength thereof is relatively poor and that it is difficult to make it into a display of large size.
Meanwhile, the aforementioned Kerr effect is also recognized in a liquid crystal. In particular, an isotropic phase thereof immediately over a nematic phase-isotropic phase transition temperature exhibits a Kerr effect which is equal to or higher than that of the PLZT. This high Kerr effect is called abnormal Kerr effect. This abnormal Kerr effect is assumably ascribed to the presence of the short-distance order of nematic molecular orientation in the isotropic phase. A liquid crystal shutter as well as a display which makes the most of Kerr effect are high in speed and free from the problems of insufficient mechanical strength of limited display area that have been accompanied with a display element employing the PLZT as mentioned above. Additionally, a liquid crystal shutter as well as a display which makes the most of Kerr effect are advantageous also in terms of viewing angle as compared with other kinds of liquid display system. However, the temperature dependence of Kerr effect becomes a serious problem in the utilization of this Kerr effect in a liquid crystal.
Generally, the Kerr constant of liquid crystal can be represented by a formula: K=A/(T-T*) (wherein A is a constant, and T* is approximately equal to a liquid crystal-isotropic phase transition temperature). As means to solve this problem of temperature dependence, the utilization of a nematic liquid crystal/a polymer composite has been proposed by Kikuchi et al (Kyushu University). Namely, the results of a test employing poly(isobutyl methacrylate) as a polymer material are reported by Kikuchi et al (Japanese Chemical Society, a draft for the 72nd Spring Meeting, pp. 226; or a draft for the 22nd Liquid Crystal Forum, pp. 413). However, the improvement of temperature dependence achieved therein is as small as several degrees centigrade at most, and there is still a problem that the Kerr constant may be markedly decreased with respect to a system consisting only of a liquid crystal.
On the other hand, it is also reported by Matsumoto et al (NTT) that if the diameter of droplet of liquid crystal in a nematic liquid crystal/polymer composite is set to 0.1 .mu.m or less, a scattering state of liquid crystal can be controlled, and that the intensity of the optical anisotropy of a nematic liquid crystal/polymer composite to be induced by an electric field is proportional to a square of electric field, the response speed thereof being 10 .mu.s or less (Appl. Phys. Lett., 69, 1044 (1996) and AM-LCD97, pp. 33). However, the Kerr effect is not referred to at all in this document. In particular, since the liquid crystal droplet having a diameter of 0.1 .mu.m or less is realized by setting the content of liquid crystal to 40% by weight or less, the polymer region which is not optically responsive is caused to increase to 60% by weight or more, thus deteriorating the contrast of the element. Additionally, the temperature characteristics of the element is not referred to at all in this document. Accordingly, even if the Kerr effect is intended in this document, how degree the temperature dependence of Kerr effect can be improved is not clear in this document.
As mentioned above, even though there have been various attempts to make use of the Kerr effect of liquid crystal, it is still difficult to sufficiently improve the temperature dependence of Kerr effect. Namely, up to date, it still fails to fully make the most of the advantage of liquid crystal that the Kerr constant thereof is large.
Additionally, it is also desired to further enhance the light utilization efficiency in a liquid crystal shutter utilizing the Kerr effect.